In the sheltered shores of tropical and subtropical coastlines in the gently sloping intertidal zones thrive the most unique wetland ecosystem of the mangroves. With the habitats characterized by high temperatures, fluctuating salinity, alternating aerobic and anaerobic conditions, periodic wet and dry environments, and an unstable and shifting substratum, mangroves have developed specialized structures which enable them to occupy the special niche between land and sea.
Perhaps the most conspicuous and characterizing adaptation of mangroves is their development of aerial roots. As the soft sediment in which they grow are usually poorly drained, rich in organic matter, and is always saline to some degree, an extensive root system is necessary to provide stability, and obtain the oxygen which are vital for plant growth. Despite belonging to many different families mangroves have come up with surprisingly similar solutions to overcome this obstacle. The Avicennia marina grows a series of snorkels or pneumatophores which project through the sediment surface to aid gas exchange between underlying roots and the atmosphere. Stilt/prop roots grow downwards from the trunk and branches, providing gas exchange in the exposed sections and water uptake in the sections below the sediment and are characteristic in Rhizophora stylosa and Ceriops tagal. Knee roots are raised loop sections of cable roots, with thickening on the upper exposed side, forming 'knees'. These occur in Brugueira gymnorrhiza and Ceriops tagal. Some species, however, such as Aegiceras corniculatum do not have aerial roots, but have other adaptations to aerate the root system, such as a spongy, enlarged stem base.
Most plants have a very low tolerance for salt, but in mangroves, twice a day, the high tide rushes in and covers the majority of plants in saltwater. To defend themselves against the daily onslaught of salt, mangroves exclude salt by filtering it out at the roots; Rhizophora stylosa, Brugueira gymnorrhiza and Ceriops tagal are all salt excluders.
